Brake pedal travel


#1

I have a '68 GMC pickup which was totally rebuilt for the quarter-mile track by a previous owner. Frame strengthening, extensive power train upgrades, etc. The P/U must have had a disk brake upgrade. I have excessive brake pedal travel, and the harder I push the brake pedal the better the brakes stop the P/U: the brakes will not lock up regadless of how hard I press the brake pedal. I think it is one of two problems: there is a brake pedal travel adjustment that isn’t shown in the Chilton referance, or the power brake (Vacuum) booster is bad. At high RPM/Idle there is a loud squeel coming from the engine compartment which sounds like an intake leak - it is not. Could this squeel that I am hearing be the booster being worn out? I have the proper amount of vacuum going to the booster. Any ideas anyone? Oh, I currently use this P/U as a grocery getter and it pulls a 28ft travel trailer.


#2

I would guess it was the booster, but you could have a trusted mechanic check the thickness of the disks to see if they are out of specs, worn out.

I don’t recall any adjustment for the brake pedal travel.


#3

1/2 ton, 3/4 ton or what?? Pulling a 28’ trailer with a 40 year old truck set up for drag racing is a real leap of faith…I hope the builder knew EXACTLY what he was doing, especially with the brake set-up. Good Luck…


#4

Well, in your haste to be critical you made your inexperience obvious. The obvious advantage to having this 40 year old total-rebuild is very obvious to anyone who knows what the hell they’re doing with cars. One obvious thing being that this 40 year old car will probably out last anything in curent production - including Toyota (Just wanted to make that obvious for you.). Do yourself a favor and don’t reply to anyone who communicates experience from the start. You’ll just be showing your ass.


#5

What Chilton book are you using? It sounds like this is probably not an original braking system, so the Chilton’s for a '68 pickup is going to be of marginal usefulness. If it’s just a front-disc conversion and still has rear drums, those might very well need adjustment. You have “pedal travel” in your title, but you don’t mention it in your post! How is it? How far down does the pedal go before the brakes start to grab? Can you put the pedal to the floor? Have you tried bleeding the system?

Also, the lock-the-brakes-up test probably isn’t the most scientific braking system test. What kind of tires does this thing have on it? If it has big fat tires for drag racing, they probably have enough traction on pavement that you may not be able to lock them up, especially if the brakes are just a factory-style front-disc conversion. I had a chevy PU of a similar vintage with factory discs in the front and it’s braking was just barely adequate with an empty truck and I couldn’t imagine towing anything of substance with it.


#6

I don’t think Caddyman’s beef is with the age or make of your truck, but with the fact that it was built for drag racing, which is a very different application from towing a large trailer. I’m sure this thing has plenty of power, but the brakes are probably designed for short spurts of high-speed braking with an empty truck and not for trying to hold back a couple tons of trailer going down a long downgrade.

And Caddyman is absolutely right that you’re putting a lot of faith in the person who built this thing. Drag racers tend to put the priority on power, and brakes very often wind up as a secondary concern. A disc-brake upgrade could mean a high-quality aftermarket braking system with modern performance and safety features, but it could just as easily mean just pulling a couple of disc brakes off of a slightly newer truck, in which case everything else in the system is the same and, like I mentioned below, the 1970’s-style disc-brakes aren’t that much better than 1960’s-style drum brakes.


#7

Norm, you make me laugh.
Would you describe for us please your extensive experience with vehicles built for drag racing and their braking systems?


#8
 "the harder I push the brake pedal the better the brakes stop the P/U: the brakes will not lock up regadless of how hard I press the brake pedal. "

Forgive my ignorance but what is the problem? Do you really want the brakes to lock up?


#9

I can’t believe how hyper-critical this forum is getting. 7 replies and only a couple that are marginally useful.

Norm: You need to figure out what was done to the brake system. A common mistake that is made with upgrading from drums to discs on old rigs such as this is leaving the stock brake master cylinder in place. Often times, disc brake calipers have a considerably larger volume and travel than do drum brakes. As such, you can run out of pedal before the caliper is providing full clamping force. I ran in to this issue when I was converting my '72 F-100 to Chevy style disc brakes, and had to use a master cylinder out of a much newer/larger truck. The result was braking in a half-ton truck with 35" tires that rivaled that of many newer cars.

My advice is to figure out what components you have in your braking system, or if you know, post them back here for us. You’re going to need to know anyway, because some day you will need to replace one or more of them. Best of luck, my friend.


#10

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